Since a double-layered, small molecular, green light-emitting organic electroluminescent device (OLED) was firstly developed by Tang, et al., of Eastman Kodak in 1987, which comprises TPD/Alq3 as a light-emitting layer and a charge transport layer, the development of organic electroluminescent devices has been rapidly achieved, and thus organic electroluminescent devices have currently reached commercialization. At present, an organic electroluminescent device usually uses phosphorescent materials having high luminous efficiency for the realization of panels. In the case of organic electroluminescent devices emitting red or green light, although organic electroluminescent devices using phosphorescent materials have been successfully commercialized, blue light-emitting phosphorescent materials have the following disadvantages: Due to the disappearance of excessive formed excitons, roll-off at high current is reduced thereby resulting in degradation of characteristics; blue light-emitting phosphorescent materials have a problem with respect to long-term storage stability; and color purity is rapidly reduced with the passage of time, and thus it is difficult to realize a full-color display.
Fluorescent light-emitting materials which are currently used also have many problems. First, when they are exposed to a high temperature in the process of the manufacture of panels, current properties may be changed in the organic electroluminescent devices thereby changing the light-emitting luminance, and due to the structural nature, luminance may be reduced according to the reduction in properties of an interface between a light-emitting layer and an electron injection layer. Furthermore, fluorescent light-emitting materials have lower efficiency than phosphorescent light-emitting materials in a material aspect. Although the efficiency improvement was attempted by certain fluorescent light-emitting materials such as the combination of anthracene-based host and pyrene-based dopant, since the materials have a large hole trap, they have a tendency to emit light at the interface with the light-emitting zone in a light-emitting layer being biased toward a hole transport layer. Such light-emitting at the interface not only reduces the lifespan of organic electroluminescent devices but also does not achieve satisfactory efficiency.
The above problems of fluorescent light-emitting materials are no longer possible to be overcome by the mere improvement of materials per se. Thus, attempts are currently being made to change charge movement properties by improving charge transport materials or solve the problems by developing optimized device structures.
Korean Patent Application Laying-open No. 10-2012-0092550 discloses an organic electroluminescent device in which a blocking layer comprising aromatic heterocyclic derivatives having an azine ring is disposed between an electron injection layer and a light-emitting layer.
Japanese Patent No. 4947909 presents a blue fluorescent light-emitting device comprising an electron buffer layer, which efficiently injects electrons to a light-emitting layer compared with Alq3 and controls the movement of electrons, and thereby prevents the reduction of driving voltage and the degradation due to the light-emitting at an interface, and improves the lifespan of the device.
Chinese Patent Application Laying-open No. CN 101870865, and International Publication Nos. WO 2013/149958 A1 and WO 2014/072017 A1 disclose spiro[fluorene-9,9′-xanthene] derivatives.
However, none of the above literature disclose organic electroluminescent compounds in which a benzene ring of the fluorene in the spiro[fluorene-9,9′-xanthene] backbone is fused with benzofuran, benzothiophene, or indole.